CORRELATION BETWEEN STRONTIUM ISOTOPIC COMPOSITION AND TOPOGRAHY IN VOLCANIC ROCKS FROM EASTERN CALIFORNIA

The shallow Moho under the Sierra Nevada indicates that the range is not supported by a deep low-density root. Geophysical studies indicate that topography and gravity are best explained by a combination of crustal and mantle density variations. Here we show that topography is correlated with the isotopic compositions of volcanic rocks, consistent with the source of at least some of the topography arising in the mantle.

Analysis of variations in Sr isotopic compositions (Sri) of mafic lavas from eastern California reveals a significant positive correlation between Sri and elevation. Analysis was restricted to young (<8 Ma) mafic (silica <55 wt%) lavas from the Sierra Nevada, Mojave Desert, and California Basin and Range. Sri also closely tracks the elevation of the Sierran range crest; highest values (>0.706) occur in and near the region in which 4000 meter peaks are prominent, and values fall off to the north and south along with topography. The prominent northward jump in both range and basin topography near Lone Pine corresponds with a sharp increase in Sri.

The source of this correlation could lie in variations in the composition of the lithospheric column. High Sri lavas were erupted in and near the central Sierra Nevada, a region in which Cretaceous granites are abundant. Mafic magmas in the Cretaceous batholith were probably derived from enriched lithospheric mantle, which supplied both a characteristic isotopic composition (e.g., Sri ~ 0.7065) and abundant incompatible elements (e.g., K) and led to production of abundant granite and low-density crust. The high topography of the central Sierra is at least in part a result of this low density crust (Pratt isostasy), but its source was in the Cretaceous mantle. However, high-Sri Cenozoic basaltic magmatism in the central Sierra may have been triggered by delamination of pre-Pliocene mantle lithosphere, in which case upwelling of low-density asthenospheric mantle could also contribute to the high elevation of the central Sierra.